Numerous processes have been proposed for the isomerization of one or more of xylenes (meta-xylene, ortho-xylene and para-xylene) to form other isomers of xylene. In many instances, the sought xylene isomer is para-xylene due to the demand for terephthalic acid for the manufacture of polyester.
In general, these xylene isomerization processes comprise contacting the xylene isomer sought to be isomerized with an isomerization catalyst under isomerization conditions. Various catalysts have been proposed for xylene isomerization. These catalysts include molecular sieves, especially molecular sieves contained in a refractory, inorganic oxide matrix. The catalysts also contain a hydrogenation metal, such as a platinum group metal.
Due to the large scale of commercial facilities to produce para-xylene on an economically competitive basis, not only must a xylene isomerization process be active and stable, but it also must not unduly convert xylenes to other aromatics or crack the aromatic feed so as to result in ring loss. Toluene and trimethylbenzene are two of the typical co-products from an isomerization and, because a loss in C8 aromatic values results from such co-production, processes to reduce their co-production are sought. Typically, the loss in C8 aromatic values increases as the isomerization process is driven closer to equilibrium. Accordingly, to minimize the loss of C8 aromatic values, commercial facilities often suffer inefficiencies by not driving the isomerization close to equilibrium.
Catalytic processes are sought that reduce the loss of C8 aromatic values and thus reduce the co-production of toluene and trimethylbenzene and other C9 and higher aromatics while allowing closer approaches to xylene isomerization equilibrium to be achieved.
U.S. Pat. No. 4,362,653, for instance, discloses a hydrocarbon conversion catalyst which could be used in the isomerization of isomerizable alkylaromatics that comprises silicalite (having an MFI-type structure) and a silica polymorph. The catalyst may contain optional ingredients. Molybdenum is listed as one of the many optional ingredients. U.S. Pat. No. 4,899,012 discloses catalyst for isomerization and conversion of ethylbenzene containing a Group VIII metal, lead, a pentasil zeolite and an inorganic oxide binder. U.S. Pat. No. 6,573,418 discloses a pressure swing adsorption process to separate para-xylene and ethylbenzene from C8 aromatics. Included among the catalysts disclosed for ethylbenzene isomerization are those containing ZSM-5 type of molecular sieve (Al-MFI) dispersed on silica. The catalysts contain a hydrogenation metal and listed among the hydrogenation metals are molybdenum. Suitable matrix materials are said to be alumina and silica. See example 12 which uses a molybdenum-containing catalyst for xylene isomerization. U.S. Pat. No. 6,143,941 discloses selective isomerization and ethylbenzene conversion processes using catalysts comprising a zeolite, including MFI-type zeolites, a platinum group metal and an aluminophosphate binder.
U.S. Pat. No. 4,899,011 discloses a two catalyst system for xylene isomerization and ethylbenzene dealkylation in which the first catalyst, which has low ethylbenzene diffusivity, dealkylates ethylbenzene, and the second catalyst, which has a greater ethylbenzene diffusivity, effects xylene isomerization.
U.S. Pat. No. 6,280,608 discloses layered catalysts containing a core and an outer layer containing molecular sieve and catalytic metals. One of the potential uses for the layered catalyst is said to be for isomerization reactions.